1. Field of the Invention
The present invention relates to an electromagnetic wave measuring method and electromagnetic wave measuring apparatus used to measure an electromagnetic wave from an electronic device.
2. Description of the Related Art
In recent years, regulations have been established in many countries to minimize interference due to unwanted emissions generated by various kinds of electronic devices. For example, outside of Japan, standards have been drawn up by the FCC (Federal Communications Commission) in the United States, the CISPR (International Special Committee on Radio Interference), the VDE (Verband Deutscher Elecktrotechniker), and so forth. In Japan, a voluntary regulatory standard has been decided upon by the VCCI (Voluntary Control Council for Interference by Information Technology), and electronic equipment manufacturers are undertaking measures to suppress unwanted emissions generated by various kinds of electronic devices.
In the electromagnetic wave measuring method according to this standard, field strength is measured at a position a predetermined distance from an electronic device that is a source of unwanted emission, and a special measuring environment and measuring instrument are used, so that considerable expertise is necessary in terms of the measuring technology. Also, the electromagnetic wave measuring method according to this standard measures an electromagnetic wave at a distance of 3 [m] or 10 [m] from a measured object.
Meanwhile, ascertaining the state of electromagnetic waves generated by various kinds of circuit operation in logic circuits, which have been increasing in speed in recent years, and radio communication devices incorporating various functions, has become important in improving signal quality. In electronic devices such as radio communication devices, in particular, waves that are a multiple of the circuit operating clock frequency are a factor causing degradation of radio communication quality. Furthermore, waves that are a multiple of the circuit operating clock frequency lead to such problems as malfunction due to mutual signal interference. This is referred to as an autotoxication problem (whereby a device's own operating clock affects the device's own operational functions).
This autotoxication problem is extremely difficult to solve in the case of the aforementioned standard measuring method that measures a distance. This is because, with distance measurement, interference problems within a circuit and the behavior of an extremely near electromagnetic field cannot be observed, making it difficult to identify where the source of an electromagnetic wave is located in an electronic device.
Thus, many electromagnetic wave measuring apparatuses have been proposed and commercialized whereby, for example, scanning measurement of a near electromagnetic field of an electronic device is performed using an electric field sensor or magnetic field sensor, and the source of an electromagnetic wave is estimated by measuring the near electromagnetic field distribution of a measured object. An example of a near electromagnetic field distribution measuring instrument is one that measures two-dimensional electromagnetic field distribution by scanning a plane in the proximity of a measured object, identifies a large part of a radiated electromagnetic field from the distribution image, and performs evaluation of the electromagnetic field radiated from the measured object based on the identification result (see Unexamined Japanese Patent Publication No. 2000-74969, for example).
There is also a measuring apparatus that improves the precision of near magnetic field measurement by performing calibration of a magnetic field probe using a reference antenna (see Unexamined Japanese Patent Publication No. 2000-214198, for example).
However, a conventional near electromagnetic field measuring instrument requires from several minutes to several hours to perform a scan with an electromagnetic field probe in the vicinity of a measured object (electronic device). Therefore, while useful for analysis in the product development stage, a problem with a conventional near electromagnetic field measuring instrument is the extremely high cost incurred if used in a mass production plant that produces tens of thousands of products a day.
Also, the reception sensitivity of an electromagnetic field probe used in a near electromagnetic field distribution measuring instrument decreases the lower the frequency, and reception is difficult for low frequencies on the order of tens of kilohertz. Another problem is thus the possibility of not being able to receive all the signals present in an electronic device.
A method has been proposed whereby a plate-like antenna having a plurality of sensor arrays is positioned close to a printed wiring board that is a test object, and whether or not there is a defect in the printed wiring board is detected by detecting an electromagnetic wave radiated from the printed wiring board (U.S. Pat. No. 5,218,294). Furthermore, an electromagnetic noise measuring apparatus has been proposed that identifies a noise generation location in a measured electromagnetic noise generating body by positioning a microstrip array in which a plurality of microstrip coils are arranged opposite the measured electromagnetic noise generating body (Unexamined Japanese Patent Publication No. HEI 10-104294).